Drive devices of vehicles, comprising manual transmissions having two sub-transmissions or functionally comparable, separate transmission trains, which can usually be shifted automatically or in an automated manner, are already known. Certain functions of such a drive are usually assigned to only one of the two sub-transmissions, or can only be implemented thereby, and therefore asymmetrical loads can occur on the sub-transmissions, or certain functions are mutually exclusive or are available only with limitations.
In a double clutch transmission, for example, usually one of two input-side power-shift transmissions is also always used as a start-up clutch, since the preferred start-up gear is located in the applicable sub-transmission. If the drive is subject to frequent start-up procedures, as is the case with a city bus, for example, this can result in thermal overload with premature wear of the applicable clutch.
In a hybrid drive derived from a double clutch transmission or the like, functionalities assigned to the sub-transmissions can make certain limitations necessary. For example, it is not possible to switch from a generator mode of an electric machine assigned to one of the sub-transmissions into a start-up procedure without delay. In addition, operation in a creep mode, i.e., driving at a constant low speed, is usually not possible in a generator mode. Further limitations can result when starting the internal combustion engine in an electric driving mode. In order to start the internal combustion engine and retain the tractive force in the drive train, it is usually necessary to utilize a second electric machine as a starter-generator, or an additional friction clutch, which must be designed large enough for frequently recurring start-up procedures.
In the case of vehicle transmissions subject to high loads on the start-up and braking devices thereof, such as automatic power-shift transmissions for city busses, for example, in the case of which start-up and braking procedures occur frequently and in close succession, it is known to equip such vehicle transmissions with a hydrodynamic torque converter as a wear-free start-up element and, possibly, with an additional wear-free constant-braking device, such as a hydrodynamic retarder. In such hydrodynamic transfer elements, the mechanical energy of a drive shaft is converted into the kinetic energy of a fluid, with generation of heat, and back into the mechanical energy of an output shaft. A simple fluid coupling comprising a driving impeller and a driven turbine functions as a continuously variable transmission having different speeds of rotation on the input side and the output side. In a torque converter, an additional stator redirects the flow in the direction of the impeller and thereby increases the torque in the torque conversion. In a retarder, a fixed turbine blade wheel, the stator, generates a braking effect on the driving wheel, the rotor, and thereby induces a braking effect on the drive train.
Start-up retarders mentioned above are known for the purpose of minimizing the required design complexity and the production costs, the start-up retarders combining the functions of a hydrodynamic start-up element, such as a fluid coupling or a torque converter, and a hydrodynamic retarder, in one unit.
A hydrodynamic clutch comprising an impeller and a turbine is used in such a drive system known from DE 100 45 337 A1, wherein the impeller is connected to a drive motor and a friction clutch is engaged in parallel in order to lock up the impeller and the turbine, and wherein the turbine is connected to a transmission input of a downstream manual transmission by means of a freewheel, and can be fixed on a housing by means of a turbine brake. At start-up, power is transferred to the transmission input by means of the hydrodynamic circuit. In braking, the turbine is fixedly braked and the friction clutch is engaged. Filling the hydrodynamic clutch with a fluid permits the hydrodynamic clutch to function as a primary retarder.
EP 0 879 370 B1 discloses a transmission unit comprising a hydraulic transmission part, which has a primary blade wheel and a secondary blade wheel, which together form a working chamber, which can be filled with fluid, and comprising a mechanical transmission part disposed separately downstream thereof relative to the drive train, as the actual vehicle transmission. The mechanical transmission part can be, for example, a planetary transmission having one or more coupled planetary gear sets and a plurality of forward gear steps and reverse gear steps. The hydraulic transmission part can be operated in two operating states, namely in a driving state as a hydrodynamic clutch and in a braking state as a hydrodynamic retarder. In a start-up procedure, the primary blade wheel functions as an impeller, and the secondary blade wheel functions as a turbine. In a braking procedure, the primary blade wheel is fixed and functions as a stator, and the secondary blade wheel is connected to the transmission and then functions as a rotor rotating in the reverse direction due to the opposite direction of flow. The hydraulic transmission part is assigned a plurality of shift elements, which, together with further shift elements of the mechanical transmission part, act on each of the blade wheels and couple one of the blade wheels or both blade wheels to the transmission, or bypass or fix one of the blade wheels or both of the blade wheels. Vehicle operation is implemented in each case by means of one forward gear step or one reverse gear step with the start-up retarder disengaged or bypassed. Braking action is implemented in each case by means of one reverse gear step with the primary blade wheel fixed.
DE 198 17 865 A1 discloses a start-up retarder comprising a hydrodynamic retarder and a planetary gear set, which form one unit. The start-up retarder makes it possible to implement a hydrodynamic start-up procedure with an additional start-up transmission ratio and hydrodynamic braking action. The retarder comprises a rotatable rotor blade wheel and a fixed stator blade wheel. The rotor is connected or connectable to a sun gear of the planetary gear set and is connected or connectable to a planet carrier. The planet carrier is connected or connectable to an engine-side drive shaft or to a transmission-side output shaft of the planetary gear set. A ring gear is therefore connected either to the output shaft or to the drive shaft. The retarder is designed as a so-called double-flow retarder. This comprises two flow circuits having two outer, axially opposed stators, each of which has inwardly facing blades, and an axially inner rotor having blades mounted on both sides. The rotor can therefore generate sufficiently high braking power in both of the possible rotational directions thereof.